These before-and-after images courtesy of Planet show the Punggye-ri test site, where on September 3, 2017 North Korea claimed to have conducted the underground explosion of a hydrogen bomb. (Via AFP)

More than 200 people are feared to have lost their lives after a tunnel collapsed at North Korea’s nuclear test site early last month, a report says.

According to a news report broadcast by the Japanese television network Asahi on Tuesday, some 100 workers were feared dead after the tunnel caved in at the Punggye-ri Nuclear Test Facility, based underneath the 2,200-meter-high Mount Mantap in the northeast of North Korea.

The incident occurred early last month, a few days after Pyongyang conducted its latest and largest nuclear test, a hydrogen bomb, on September 3, it added, citing unnamed North Korean sources.

Shortly after the first incident, dozens of other people were also feared dead in another tunnel collapse during rescue operations at the same site, leaving more than 200 people feared dead in total, the Japanese broadcaster went on to say, adding that the tragic accidents were triggered by nuclear detonation.

The hydrogen bomb, according to assessments made by Japanese experts, was eight times more powerful than the Little Boy, a uranium gun-type bomb that the US detonated on Hiroshima in 1945.

Earlier this month, The Washington Post published a report that Mount Mantap appeared to have suffered serious geological damage, known as “tired mountain syndrome,” which may eventually cause the peak to collapse, totally or partially.

Chinese geologists have already warned that further blasts could eventually make the mountain collapse completely, releasing radiation.

Pyongyang has already said it is considering testing a hydrogen bomb over the Pacific Ocean in response to Washington stepping up economic sanctions against the country.

North Korea is under mounting international pressure over its missile and military nuclear programs and has been subjected to an array of sanctions by the United Nations. However, Pyongyang says it needs to continue and develop the programs as a deterrent against hostility by the US and its regional allies, including South Korea and Japan.